Thermodynamic model of aqueous CO 2–H 2O–NaCl solutions from −22 to 100 °C and from 0.1 to 100 MPa

Abstract

Experimental measurements of the solubility of CO 2 in aqueous NaCl solutions have been assembled from 21 literature studies and tested for consistency using analytical and thermodynamic criteria. Of the 508 data compiled, 170 data (33%) were discarded and 36 were reserved to test the high-pressure dependency of the model. Possible reasons for the observed discrepancies between datasets are discussed. The 302 measurements that satisfy the acceptance criteria have been used to fit Pitzer parameters. These have been incorporated into a semi-empirical, γ– ϕ type thermodynamic model that builds upon published equations of state for the unary and binary subsystems. The accepted experimental solubilities are reproduced by the model with a precision of better than 1.6% (one standard deviation) over the entire P– T– x range considered. The new model provides a thermodynamically consistent description of numerous properties of the aqueous liquid in the ternary CO 2–H 2O–NaCl system, including: the activity coefficients, activities and partial molar volumes of CO 2(aq) and Na +Cl − (aq); the activity coefficient and osmotic coefficient of the solvent H 2O; the Setchenow coefficient; saturation indices of all four solid phases in the system (CO 2–clathrate–hydrate, ice, hydrohalite and halite); and the molar volume, excess molar volume and density of the bulk liquid. These properties can be calculated for any CO 2 concentration up to saturation, and for any NaCl concentration (whether stable or metastable). The model is available as a computer code at < www.geo.unibe.ch/ diamond>.